Fix the branded navbar

[petrbroz]

In da171eb we've replaced the default Docusaurus navbar with a custom one from the APS dev. portal team. Currently this can only be done by importing a complete JavaScript bundle which - together with all its dependencies (incl. React) - is over 3MB in size. That's not good.

Let's work with the APS dev. portal team and make sure they provide us with a React Component that we can cleanly insert into this Docusaurus project.

[petrbroz]

@augustogoncalves since you're the one talking to the dev. portal team, could you please lead the effort on this?